Touch sensitive panels and screens are commonly used in many types of computerized equipment. In some systems, a touch screen avoids the need for providing a keyboard.
Referring to FIG. 1, a touch screen is typically used to allow the user to easily select one of a plurality of displayed items. The user makes his or her selection by touching the portion of the screen associated with the item to be selected. To clarify the boundaries of the areas associated with each item, the image on the screen may include boxes surrounding the displayed items.
The uses of touch screens have grown increasingly sophisticated, allowing the user to draw pictures, manipulate menus, use a displayed keyboard for alphanumeric input, and so on.
The terms "screen", "display", and "panel" are used synonymously herein. The present invention concerns the touch aspect of touch screens. Therefore, for this purpose it is unimportant how the image on the touch sensitive apparatus is displayed. The touch mechanism could even be used with a static image instead of with a display device.
In most cases, the term "touch sensitive" is a misnomer. Most touch sensitive screens, including the present invention, sense the interruption of one or more light beams; they do not sense actual physical contact with the screen or panel. The display is surrounded by pairs of light emitting and light sensing elements, such as infrared light emitting diodes. These pairs are individually enabled in a preselected pattern, and the position of any object (such as the user's finger) touching the screen is determined by looking at which lights are blocked by the object. For this reason the phrases "touching the display" and "adjacent to the display" are used synonymously herein.
A serious shortcoming in prior art touch screens is that the prior art screens cannot distinguish between a person deliberately touching the screen with one pointed finger and accidental or inadvertent touchings or actuations of the screen--such as by an insect landing on the screen, a wad of paper bouncing against the screen, a box or document being pushed up against the screen, or even a person accidentally brushing up against the screen. This is a particularly significant problem for horizontally oriented screens, because it is easy for an object (e.g., a pencil or a book or a memorandum) to fall onto a horizontal screen.
Another significant problem in prior art touch screens is that the prior art screens are totally disabled when a single beam element (i.e., a light beam emitter or a light beam detector) fails. This is because a component failure looks the same as a beam blocked by a person's finger: both result in the failure to detect a particular beam going across the screen.
Both of these problems with prior art touch screens have made touch screen more prone to failure than other computer interfaces, and have therefore limited the range of applications in which touch screens are used.
It is therefore a primary object of the present invention to provide solutions to both of the above identified problems. As a result, the present invention provides a fault tolerant touch screen which continues to operate even after one or more beam components have failed. The touch screen of the present invention is also able to distinguish between deliberate user selections and accidental or inadvertent touchings of the screen.